The present invention is directed to regenerating the resins used for polishing water, such as water used in steam turbine power plant systems.
In many applications, such as for example in steam turbine power plant systems, cation exchange resins and anion exchange resins are used in a polishing system to remove anionic and cationic solid forming precursors such as sodium, calcium, magnesium, sulfate, and chloride ions from the water. If not removed, these solid forming precursors could produce corrosion within the turbine, boiler and conduits.
Most commonly, such resins are employed as mixed beds. In operation, the resins become spent and must be regenerated. For purposes of regeneration, the resins are backwashed to remove solids filtered out of the system water by the resins, and the resins are separated. The anion resin is regenerated with an alkaline material, usually sodium hydroxide and the cation resin is regenerated with an acid, usually sulfuric acid. Techniques for separating the resins and regenerating the resins are described in U.S. Pat. Nos. 3,385,787 to Crits, et al.; 3,501,401 to Calmon; 3,582,504 to Salem, et al.; 3,709,818 to Gustafson, et al.; and 3,414,508 to Applebaum, et al., all which are incorporated herein by this reference.
A commonly used technique is to ammoniate the beds by recycling an aqueous solution of ammonia between the tank containing the cation exchange resin and the tank containing the anion exchange resin. It has been found that ammoniation during regeneration helps maintain a proper ammonia content in the water to minimize metal corrosion, allows for long runs between regeneration, and removes sodium from the exchange resin during regeneration.
Generally, the polisher is operated until "ammonia breakthrough". The presence of relatively large quantities of ammonia in the effluent from the polisher indicates that the bulk of the cation exchange resin has been used up so that residual sodium left on the cation exchange resin during regeneration is displaced by ammonia from the cation exchange resin.
However, this prior art regeneration process has proven to be inadequate for at least once-through supercritical steam turbines, when the polisher was operated beyond ammonia breakthrough. Corrosion has occurred in the last stages of the low pressure main turbines and in the wet zones of auxiliary turbines, as well as in the reheaters. The result has been extensive pitting as well as stress corrosion cracking of turbine blades, which has required their replacement.
In view of this problem, there is a need for a process for regenerating the cation and anion exchange resins used to polish water used with steam turbine generators that not only removes solid precursors from the process water, but also removes from the water the agents contributing to this severe corrosion problem.